1. Field of the Invention
The present invention relates generally to a thermal printing apparatus for transferring by thermal printing sound signals, video signals and so on recorded on a master magnetic tape to a slave magnetic tape, and more particularly is directed to a thermal printing apparatus which includes a device which measures temperature of the slave tape and controls the transport speed of the slave and master tapes to accurately control the temperature. The difference between the measured temperature of the slave tape and a reference temperature is used to control the transport speed and thus, achieve optimum thermal printing of signals on the slave tape from the master tape.
2. Description of the Prior Art
At present, a thermal printing method has been known for transferring signals recorded on a master magnetic tape to a slave magnetic tape. In the prior art method, a magnetic tape which uses chromium dioxide CrO.sub.2 as a magnetic material is used as a slave magnetic tape. In this case, the Curie temperature of the chromium dioxide is about 130.degree.C which is relatively low and the coercive force of the chromium dioxide greatly depends upon the temperature. The coercive force of the chromium dioxide is about 500 O.sub.e at room temperature, but decreases as the chromium dioxide is heated. When the chromium dioxide is heated to about 130.degree.C, its coercive force becomes zero. When using a slave tape having the characteristics mentioned above, it is desirable to use a master tape with magnetic material having a higher Curie temperature than that of the slave tape and when the slave tape is heated to about 130.degree.C, and thereafter is cooled so that information signals recorded on the master tape will be transferred to the slave tape.
In one of the embodiments of prior application, Ser. No. 444,571, when the temperature of the slave tape deviates from a reference temperature (about 130.degree.C), a heating device such as a lamp is controlled in brightness as a function of the difference between the temperature of the slave tape and the reference temperature so as to make the temperature difference substantially zero. However, this requires a long interval of time and thus the speed of response is slow and hence the thermal printing temperature cannot be accurately maintained.